Pyreno[4,5-b]furan and pyreno[4,5-b:9,10-b]difuran derivatives as new blue fluorophores: Synthesis, structure, and electronic properties

Article abstract of DOI:10.1246/cl.131198

New pyreno[4,5-b]furan and pyreno[4,5-b:9,10-b]difuran derivatives displaying intense blue fluorescence in CH2Cl2 were efficiently synthesized through the reactions of pyrene-4,5-dione and 2,7-di-tert-butylpyrene-4,5,9,10- tetraone with benzylphosphoranes as a key step. X-ray crystallographic analysis reveals that both pyrenofurans have a planar structure, and their electrochemical properties obviously indicate high HOMO energy levels as predicted by density functional theory (DFT) calculations.

Full text of DOI:10.1246/cl.131198

CL-131198
Received: December 20, 2013 | Accepted: January 15, 2014 | Web Released: January 21, 2014
Pyreno[4,5-b]furan and Pyreno[4,5-b:9,10-b¤]difuran Derivatives as New Blue Fluorophores:
Synthesis, Structure, and Electronic Properties
Taketo Kojima,1 Ruri Yokota,1 Chitoshi Kitamura,1,³ Hiroyuki Kurata,2,³³ Mirai Tanaka,3 Hiroshi Ikeda,3 and Takeshi Kawase*1
1Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo,
2167 Shosha, Himeji, Hyogo 671-2280
2Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043
3Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531
(E-mail: kawase@eng.u-hyogo.ac.jp)
New pyreno[4,5-b]furan and pyreno[4,5-b:9,10-b¤]difuran
derivatives displaying intense blue fluorescence in CH2Cl2 were
efficiently synthesized through the reactions of pyrene-4,5-dione
and 2,7-di-tert-butylpyrene-4,5,9,10-tetraone with benzylphos-
phoranes as a key step. X-ray crystallographic analysis reveals
that both pyrenofurans have a planar structure, and their
electrochemical properties obviously indicate high HOMO
energy levels as predicted by density functional theory (DFT)
calculations.
Treatments of pyrene-4,5-dione (6) with benzylphospho-
ranes 3a and 3b produce pyreno[4,5-b]furans 4a and 4b in 80%
and 46% yields upon dehydrogenation with 2,3-dichloro-4,5-
dicyano-p-benzoquinone (DDQ) (Scheme 2).7 On the other
hand, because of poor solubility of pyrene-4,5,9,10-tetraone
(7a) in common organic solvents, the reaction of 7a resulted in
failure. Then, the reaction of 2,7-di-tert-butylpyrene-4,5,9,10-
tetraone (7b) as a starting material afforded pyrenodifurans 5 in
35% yield as a mixture of two stereoisomers (anti-5:syn-5 =
ca. 1:1). Because of good crystallinity, the anti-5 can be easily
separated in pure form as prismatic crystals with faint pink color.
On the other hand, only a small quantity of syn-5 can be
collected as colorless fine needles upon sequential fractional
recrystallizations.
Good single crystals of 4a and anti-5 for X-ray crystallo-
graphic analysis were obtained.7-9 ORTEP drawings of 4a are
shown in Figure 1. The pyrenofuran moiety of 4a possesses a
highly planar structure in the crystal. The pyrene skeleton of 4a
is slightly perturbed by the fusion of furan rings; the bond length
of the furan-pyrene fusion (C4-C5) of 4a (1.374 ¡) is longer
Pyrene is an important aromatic hydrocarbon and has been
recently the subject of numerous investigations. Due to its high
abilities of fluorescence and charge-transport, pyrene is widely
used as a building block for molecular materials for organic
electronics.1,2 Recently, polycyclic conjugated systems contain-
ing furan moieties have been explored for the design of organic
semiconductors.3 Therefore, fusion of pyrene and furan units
into one molecule would generate structurally new materials
with novel physical properties. However, only a few compounds
in this category have been reported so far.4 In 1969 Sullivan and
co-workers reported the synthesis of 9,10-diphenyl- and 9,10-
bis(4-bromophenyl)phenanthrofurans 1 (a: Ar = Ph, b: Ar = p-
BrC6H4) by the reaction of 9,10-phenanthroquinone (2) with
2 equivalents of benzylphosphoranes 3a and 3b as a key step
(Scheme 1);5 however, the reaction has not been applied to
other fused furan systems thus far. This time we found that the
reaction provides pyreno[4,5-b]furan and heretofore unknown
pyreno[4,5-b:9,10-b¤]difuran6 derivatives 4 and 5. Here we
report the structure and electronic properties of the pyrenofurans.
2 equiv 3a or 3b
1)
O
O
DMSO
4a
: Ar = C6H5 80%
4b: Ar = p-BrC6H4 46%
2) DDQ, Toluene,
70 °C, 4 h
6
R
2 equiv 3a
1)
O
O
O
O
DMSO
anti-5 + syn-5 35%
2) DDQ, Toluene
70 °C, 4 h
R
2 equiv ArCH=PPh3 (3)
1)
O
O
O
DMSO
7a
: R = H
7b: R = tBu
Ar
2) NBS
Ar
3a
: Ar = C6H5
3b: Ar = p-BrC6H4
1a
: Ar = C6H5
1b: Ar = p-BrC6H4
Scheme 2. Synthesis of 4, and anti-5, and syn-5.
2
tBu
tBu
Ph
10
10
9
4
O
4
5
10
9
4
5
O
O
O
Ar
Ph
Ph
Ph
Ph
O
9
5
Ar
Ph
Ph
Ph
tBu
syn-5
4a
: Ar = C6H5
4b: Ar = p-BrC6H4
tBu
anti-5
Scheme 1. Synthesis of phenanthrofurans 1 from 2, and
molecular structures of pyreno[4,5-b]furans 4 and anti- and
syn-pyreno[4,5-b:9,10-b¤]difurans 5.
Figure 1. ORTEP views of 4a showing 50% thermal ellip-
soids. Top view (left) and side view (right).
696 | Chem. Lett. 2014, 43, 696–698 | doi:10.1246/cl.131198
© 2014 The Chemical Society of Japan

Table 1. Optical data of 1a, 4a, 4b, anti-5, and syn-5
e
ΦSD
a
b
c
d
Comp.
-
AB,max
(log ε)
-
¦
SS
ΦSN
EM,max
f
(-EM,max
)
1a
329 (4.41)
363 (3.76)
308 (4.60)
332 (4.32)
365 (4.29)
383 (3.80)
309 (4.59)
336 (4.44)
365 (4.37)
385 (4.06)
298 (4.63)
314 (4.74)
369 (4.63)
391 (4.22)
337 (4.65)
394 (3.75)
374
391
386
408
432
810 0.31
270 0.55
0.46
(420)
0.10
4a
4b
(457)
387
410
438
135 0.80
259 0.73
190 0.57
0.13
(433)
Figure 2. ORTEP views of anti-5 showing 50% thermal
ellipsoids. Top view (up) and side view (bottom). In the side
view, t-butyl groups are omitted for clarity.
anti-5
syn-5
395
417
441
0.07
(422)
than those of C9-C10 bonds of 4a (1.348 ¡) and pyrene itself
(1.341 ¡).7,10 While α-phenyl group of 4a twists at 22.1° from
the pyrenofuran plane, the β-phenyl group does at 65.0°. The low
dihedral angle between α-phenyl group and pyrenofuran plane
prompts extension of the π-conjugation. In the crystal, 4a forms a
dimeric structure with slipped parallel stacking, but it does not
construct a columnar structure.7 Figure 2 shows ORTEP draw-
ings of anti-5 possessing Ci symmetry in the crystal. The bond
length of the furan-pyrene fusion (C4-C5 and C9-C10) of anti-5
(1.372 ¡) is almost the same as the corresponding bond of 4a.
The twisted angle of β-phenyl groups (77.1°) of anti-5 from the
pyrenodifuran plane is also considerably larger than that of α-
phenyl group (20.9°). The presence of two tert-butyl groups and
highly twisted β-phenyl groups hamper the π-stacking of the
molecules, indicating that little π-π interaction is operative
between the molecules of anti-5 in the crystal.7
Pyrenofurans 4 and 5 show intense blue fluorescence in
CH2Cl2. The selected absorption and fluorescence spectra, and
the optical data of the pyrenofurans are shown in Table 1 and
Figure 3, together with those of 1a. The absorption spectra of 1a
and 4a coincidentally resemble syn-5 and anti-5, respectively.7
The π extension from phenanthrene to pyrene moieties increases
the intensity of fluorescence in CH2Cl2. The fluorescence spectra
of 4a and 4b are almost superimposed,7 indicating that no
fluorescence quenching by the heavy atom effect is observed in
the system; on the contrary, the quantum yield of 4b in CH2Cl2,
397
420
445
0.03
(426)
aAbsorption maxima (nm) in CH2Cl2. bFluorescence maximum
(nm) in CH2Cl2. -EX = 340 nm. cStokes shift (cm¹1) in
CH2Cl2. dFluorescence quantum yields («10%) in CH2Cl2
determined relative to 9,10-diphenylanthracene as an actino-
e
meter. Absolute quantum yields in the solid state, measured
f
using an integrating sphere. Fluorescence maximum (nm) in
crystalline powder forms. -EX = 340 nm.
6 × 104
5 × 104
4 × 104
3 × 104
2 × 104
1 × 104
0
1.2
1
1a
4a
anti–5
syn–5
0.8
0.6
0.4
0.2
0
ε
250
300
350
400
450
500
Wavelength/nm
Figure 3. Absorption (the left) and fluorescence (the right)
spectra of 1a, 4a, anti-5, and syn-5 in CH2Cl2.
ΦSN = 0.80, is the highest in all pyrenofurans presented here.
The absorption edges corresponding to the HOMO-LUMO
energy gap is red-shifted in order of increasing π extension:
phenanthrofuran 1a < pyrenofurans (4a µ 4b) < pyrenodifurans
(anti-5 µ syn-5).7 The small Stokes shifts (¦SS <300 cm¹1) and
clear appearance of vibronic structures suggest that the pyreno-
furan frameworks are rigid in CH2Cl2. When the concentrations
of pyrenofuran in CH2Cl2 increase, no fluorescence changes due
to the formation of an excimer are observed within measurable
ranges (10¹4-10¹6 M).
To gain insight into the structural and electronic properties,
density functional theory (DFT) calculations with 1a, 4a, syn-5,
and anti-5 were carried out by using the B3LYP/6-31G(d)
level of theory.7,12 The electron density of the HOMOs and
LUMOs are widely expanded around the pyrenofuran moieties
(Figure 4). The calculated HOMO-LUMO energy gaps are
consistent with the data obtained by analysis of the absorption
spectra.
The pyrenofurans 4 and 5 also show fluorescence in the
solid state, but the quantum yields (ΦSD) are considerably lower
The cyclic voltammetry of 4a and anti- and syn-5 together
with 1a was measured in CH2Cl2 in the presence of n-Bu4NClO4
(0.1 M) using Fc/Fc+ as an internal standard. One-electron-
oxidation potentials were found at +0.80 V for 1a, +0.76 V for
4a, +0.56 V for anti-5, and +0.58 V for syn-5.7 The relatively
high oxidation potentials of the pyrenofurans obviously indicate
the high HOMO energy levels, and the order is 1a < 4a < anti-
than that of 1a (Table 1).7,11 The fluorescence maxima (-EM,max
)
is red-shifted in order of 1a < anti-5 µ syn-5 < 4b < 4a. The
order is not consistent with the order observed in CH2Cl2. The
results probably follow the efficiency of π-π interaction of the
pyrenofuran moieties in the solid state.
Chem. Lett. 2014, 43, 696–698 | doi:10.1246/cl.131198
© 2014 The Chemical Society of Japan | 697

ogy, 3-6-1 Gakuen, Fukui 910-8505
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A stereoisomer, 2,7-di(tert-butyl)pyreno[4,5-c:9,10-c¤]difuran is
known: D. Franz, S. J. Robbins, R. T. Boeré, P. W. Dibble, J. Org.
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Figure 4. The HOMO, LUMO, and their energy levels of
a) 1a, b) 4a, c) anti-5, and d) syn-5 by the DFT (B3LYP/
6-31G(d)) calculations.
5 µ syn-5, as predicted by the DFT calculations. The high
reversibility of the oxidation waves of the furans except 4a
indicates the high stability of the corresponding radical cations.
On the other hand, reduction potentials of all the furans were not
observed in measurable range (> ¹2.1 V).
6
7
Experimental details of syntheses of 4a, anti-5, and syn-5, crystal
packing of 4a and anti-5, and fluorescence in the solid state, the
CV spectra and the time-dependent (TD) DFT calculations of 1a,
4a, anti-5, and syn-5 are provided in the Supporting Information
(SI). The SI is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
In conclusion, pyreno[4,5-b]furan and pyreno[4,5-b:9,10-
b¤]difuran derivatives displaying intense blue fluorescence are
successfully synthesized from pyrene-4,5-dione and 2,7-di-tert-
butylpyrene-4,5,9,10-tetraone as stable crystalline materials. The
crystallographic analyses reveal high planarity of the pyreno-
furan π systems, and the electrochemical studies indicate the
high electron-donating ability of pyrenofurans, as predicted by
the DFT calculations. We are currently investigating potential
applications of these compounds to organic electronic devices;
the results will be reported in due course.
8
9
4a: Crystal system: Monoclinic, a = 24.4561(13) ¡, b =
10.6862(6) ¡, c = 15.7095(6) ¡, ¢ = 106.1610(12)°, V =
3943.3(3) ¡3, space group: C2/c, Z = 8, ® = 0.079 mm¹1, T =
223 K, R[F2 > 2·(F2)] = 0.0482, wR2 = 0.1313, F = 1.015,
Refl/param. = 4500/280. (CCDC 980129).
Anti-5: Crystal system: Monoclinic, a = 10. 8602(5) ¡, b =
16.0684(8) ¡, c = 11.9218(6) ¡, ¢ = 105.6720(10)°, V =
This study was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (Nos. 21108520, 21108521,
23108718, and 23108720 “π-Space”) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
¹1
1897.18(16) ¡3, space group: P21/c, Z = 2, ® = 0.073 mm
,
T = 223 K, R[F2 > 2·(F2)] = 0.0700, wR2 = 0.1951, F = 1.083,
Refl/param. = 4341/248. (CCDC 980128).
10 C. S. Frampton, K. S. Knight, N. Shankland, K. Shankland,
J. Mol. Struct. 2000, 520, 29.
References and Notes
11 H. Tsuji, C. Mitsui, Y. Sato, E. Nakamura, Adv. Mater. 2009, 21,
3776.
12 The DFT calculations were performed at the B3LYP/6-31G(d)
level using Gaussian 09 program. M. J. Frisch et al., Gaussian 09
(Revision A.02), Gaussian, Inc., Wallingford CT, 2009.
³
Present address: Department of Materials Science, School of
Engineering, University of Shiga Prefecture, 2500 Hassaka-cho,
Hikone, Shiga 522-8533
³³ Present address: Department of Environmental and Biological
Chemistry, Faculty of Engineering, Fukui University of Technol-
698 | Chem. Lett. 2014, 43, 696–698 | doi:10.1246/cl.131198
© 2014 The Chemical Society of Japan